Chapter 4 Networks in Their Surrounding Contexts - Cornell University

4.4. TRACKING LINK FORMATION IN ON-LINE DATA 97
person
B
A
person
(a) Triadic closure
person
B
C
person
A
person
person
B
(c) Membership closure
A
focus
(b) Focal closure
Figure 4.6: Each of triadic closure, focal closure, and membership closure corresponds to the
closing of a triangle in a social-affiliation network.
C
focus
C
person
with that of her friend A. Continuing the analogy with triadic closure, we will refer to
this kind of link formation as membership closure.
Thus, three very different underlying mechanisms — reflecting triadic closure and aspects
of selection and social influence — can be unified in this type of network as kinds of closure:
the formation of a link in cases where the two endpoints already have a neighbor in common.
Figure 4.7 shows all three kinds of closure processes at work: triadic closure leads to a new
link between Anna and Claire; focal closure leads to a new link between Anna and Daniel;
and membership closure leads to Bob’s affiliation with the karate club. Oversimplifying the
mechanisms at work, they can be summarized in the following succinct way:
(i) Bob introduces Anna to Claire.
(ii) Karate introduces Anna to Daniel.
(iii) Anna introduces Bob to Karate.
4.4 Tracking Link Formation in On-Line Data
In this chapter and the previous one, we have identified a set of different mechanisms that
lead to the formation of links in social networks. These mechansisms are good examples